RF multiplexers are multi-port networks and are components of communication systems that support multiple frequency bands or simultaneous transmit and receive functions in a Frequency Division Duplex (FDD) scheme. High selectivity, low insertion loss, high isolation between ports, compact size, ability to handle large signals (power handling), high linearity, manufacturability, and low cost may be some of the desired features for RF multiplexers.
The RF duplexer, a three-port network, is the most common form of RF multiplexer, where the ports are nominally connected to an antenna, a transmitter, and a receiver of an FDD communication system.
The increase of number of frequency bands that are allocated to wireless communication systems, such as those used in cellular phones, has resulted in using several RF filters, switches, and duplexers in the same device. For instance, RF switches can select the appropriate RF filters and duplexers that correspond to the desired RF frequency bands.
Some wireless communication standards require simultaneous operation of the receiver or transmitter at multiple frequency bands to achieve higher capacity, diversity, or data rate. For instance, the fourth generation wireless standards, commonly referred to as 4G, envision Carrier Aggregation (CA) to increase the total bandwidth to the user. In such cases, RF multiplexers can be used.
An RF multiplexer, in its simplest form, is a 1×N network that includes 1 nominal input and N nominal output ports, where N is a positive integer. Ideally, the transfer functions from the input to each of the N output ports are non-overlapping in frequency while the N output ports are isolated at their respective frequency bands. In other words, the transfer functions from each of the output ports to every other output port should have a small magnitude at the frequency bands corresponding to those two ports.
Conventional 1×N RF multiplexers include RF Band-Pass Filters (BPF) with distinct passband frequencies that are connected to a common port using a passive network or a number of passive networks. The passive network or networks can ensure proper impedance at all frequency bands of interest and may assist in enhancing the frequency response.
The requirements for RF filters and multiplexers have become more stringent in light of new communication standards where information channels and frequency bands are closer to each other, new communication devices such as smartphones where the footprint and cost of all components must be very small as more components are needed in support of multiple standards and applications, and co-existent communication systems where multiple communication transmitters and receivers work simultaneously.
Linearity, noise, and power handling requirements might lead to utilization of passive RF filters and multiplexers in many applications. The performance of passive RF filters and multiplexers may be limited by the quality factor (Q) of the components that are used in their realization. The filter selectivity as well as passband requirement may lead to a filter topology and filter order. For a given RF filter or duplexer topology and order, insertion loss may reduce with the increase of component Q.
Various technologies can be used to realize passive RF filters and multiplexers. For instance, capacitors, inductors, or transmission lines can be used to realize passive RF filters and duplexers. Electromagnetic resonators, including waveguide resonators and dielectric resonators, can also be used to realize passive filters and duplexers. The quality factor of such components is proportional to their overall physical size. As such, it has been difficult to realize compact low-loss selective passive RF filters and duplexers using electromagnetic components and resonators.
Piezoelectric material can be used to realize compact high-Q resonators. Surface acoustic wave (SAW) resonators have been widely used to realize compact low-loss selective RF filters and duplexers as well as oscillators. More recently, bulk acoustic wave (BAW) resonators have been used to construct high-performance RF filters and duplexers as well as oscillators. Ceramic resonators and micro electro mechanical system (MEMS) resonators with high quality factor have also been used in frequency generation as well as filtering applications.
RF SAW filters and duplexers have been used widely in wireless communications such as cellular phones, wireless local area network (WLAN) transceivers, global positioning system (GPS) receivers, cordless phones, and so forth. RF SAW filters have been used as band-select filters, image-reject filters, intermediate frequency (IF) filters, transmitter noise or spur reduction filters, and so forth. A smartphone may have several SAW resonators, SAW filters, and SAW duplexers to support various communication systems and standards.
Significant research and development on BAW technology has resulted in BAW resonators that have lower loss (or higher Q) or are more compact, especially at higher frequencies, compared with SAW resonators. Therefore, RF filters and duplexers that use BAW resonators may have lower insertion loss, or higher selectivity, or smaller form factor compared with those that utilize SAW resonators especially at higher frequencies. Thin film bulk acoustic resonators (FBAR) and solidly mounted resonator (SMR) are common examples of BAW resonators.
Modern wireless communication standards designate many different operational frequency bands to support the increase in the overall wireless capacity and reach. For instance, current cellular phone standards may include RF frequency bands that span around 700 MHz to around 4000 MHz. Furthermore, in order to increase the overall wireless capacity, the frequency spacing between adjacent frequency bands or channels within the same application or different applications may be reduced. This may be done, for instance, by reducing the guard bands in wireless standard or by placing the transmit and receive frequency bands in an FDD scheme closer to each other. As a result, RF filters and duplexers with higher selectivity may be required. More selective RF filters and duplexers that utilize a given component or technology (SAW, BAW, etc.) may incur more in-band insertion loss. The higher RF filter or duplexer insertion loss may reduce the wireless receiver noise figure and sensitivity, increase the wireless transmitter power consumption or reduce the transmitted power, and/or deteriorate the overall performance of a communication system.
In commercial systems, the choice of technology may depend on the technical performance, such as power consumption as well as economic and business considerations such as cost, size, and time to market. For instance, while one technology may offer a better performance compared with another technology, it may not be adopted for a commercial system that is cost sensitive. In the case of RF filters and duplexers, it may be desirable to use a technology that leads to the lowest-cost and/or most-compact solution, as long as a predetermined performance criterion is met. In other words, a more expensive or larger solution may not be adopted, even if it offers a better performance as compared with an alternative solution that meets an acceptable performance level at a lower cost and/or size. For instance, while RF filters and multiplexers that use BAW resonators may offer lower loss compared with RF filters and multiplexers that use SAW resonators for a given set of specifications, the higher relative cost of BAW technology, as well as its relatively smaller number of suppliers, may disfavor their usage in certain applications and standards. Other considerations may be the ease of integration with the rest of the components in a communication system. For instance, there may be performance, business, or economic advantages to integrate RF filters and multiplexers with low noise amplifiers (LNA), power amplifiers (PA), transmit/receive (T/R) or band-select switches, impedance matching networks, etc. A modern wireless communication device, such as a smartphone, may have a number of SAW filters and multiplexers as well as a number of BAW filter and duplexers. Each SAW or BAW filter or duplexer may be used for a specific communication application, standard, or frequency band.
A conventional method to design acoustic resonator based filters and duplexer is to decide upon the number of resonators to be used depending on the required stopband rejection in the case of filters or the required isolation in the case of duplexers. The larger the number of resonators used in filter design, the larger may be the order of the filter and the sharper may be the filter roll-off around passband. Sharper filter roll-off may mean higher stopband rejection. Similarly, the number of resonators used in the TX and RX filters of the duplexer may determine the total isolation from TX to RX. The larger the order of the TX and RX filters (i.e., the larger the number of resonators used in them), the larger may be the amount of isolation between TX and RX. Due to the limited quality factor of the acoustic resonators, the insertion loss in the filter and duplexer may be directly proportional to the number of the resonators used. In other words, the larger the order of the filter and the TX and RX filter, the larger may be the loss of the filter and duplexer, respectively.
What is needed are architectural solutions that enable realization of highly selective low-loss multiplexers with high isolation between the ports. Specifically, it is highly desirable to use a lower cost or more compact technology within an innovative architecture that satisfies a comparable or better specification compared to what can be achieved using a more expensive or less compact technology. Examples might include replacing BAW multiplexers with SAW multiplexers using an innovative architecture, or replacing ceramic or cavity multiplexers with BAW multiplexers using an innovative architecture.
Furthermore, what is needed are architectural solutions that enable realization of tunable, reconfigurable, or programmable RF multiplexers that can satisfy the requirement of multi-standard communication systems are highly sought after.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present disclosure as set forth in the remainder of the present application with reference to the drawings.